Power storage devices include, for example, secondary batteries, such as lithium ion batteries, nickel hydride batteries, and lead batteries, and electrochemical capacitors, such as electric double layer capacitors. Due to miniaturization of mobile devices, limitation of installation spaces, or the like, power storage devices are required to be further miniaturized, and thus attention is being given to lithium ion batteries for their high energy density. Packaging materials for lithium ion batteries have conventionally been metal cans; however, multilayer films are increasingly used because they are lightweight, highly heat dissipating, and produced at low cost.
Such a lithium ion battery using a multilayer film as a packaging material employs a configuration in which battery contents (e.g., cathode, separator, anode, and electrolyte solution) are covered with a packaging material including an aluminum foil layer to thereby prevent moisture from penetrating into the battery. Lithium ion batteries employing such a configuration are referred to as aluminum laminated lithium ion batteries.
Embossed lithium ion batteries, for example, are known as aluminum laminated lithium ion batteries. Such an embossed lithium ion battery has a recess formed at part of the packaging material by cold forming, battery contents stored in the recess, and the rest of the packaging material folded back, with the edge portions heat-sealed. (See, for example, PTL 1). With these lithium ion batteries, a recess formed deeper by cold forming enables storage of more battery contents to achieve higher energy density.